Image forming system, image forming device, and image forming method

ABSTRACT

An image forming system that classifies a block formed by Nb pixels included in image data into Nt groups including a group including a pixel of a highest gradation value and a group including a pixel of a lowest gradation value, generates, for at least one of the Nt groups, positional information that is information indicating positions of pixels that form the corresponding group, determines a block gradation value in accordance with a data amount that is smaller than a data amount that is used for expressing gradation values of the Nb pixels for each of the Nt groups as a target, based on a gradation value for each of pixels that form each group, tentatively determines, determines, for the block as a target, the dot arrangement using the tentatively determined dot arrangement and the positional information, and forms an image using the determined dot arrangement.

BACKGROUND 1. Technical Field

The present invention relates to image formation.

2. Related Art

Japanese Patent No. 4375398 describes the following contents as a methodfor determining in which pixel a dot is to be formed. Pixels are dividedinto pixel groups (blocks) each including a predetermined number ofpixels, and a pixel group gradation value representative of each pixelgroup is determined. Subsequently, by referring to the correspondencerelation where the pixel group gradation values and multivalueconversion resultant values (block gradation values) determined bymultivalue-converting the pixel group gradation values are set for eachpixel group, the pixel group gradation values are converted into amultivalue format. Thus, the multivalue resultant values determined foreach pixel group are supplied as control data for an image outputdevice. The image output device determines whether a dot is to be formedfor each pixel in a pixel group, based on the supplied control data, andforms dots on an output medium to output an image. Japanese Patent No.4375050 and Japanese Patent No. 4375071 also describe similar contents.

SUMMARY

In the above-described related art, one block gradation value isdetermined for each block. Therefore, if there is an edge in a block,there may be a case in which image quality is deteriorated.

An advantage of some aspects of the invention is that, in a technologyof performing halftone processing using a block gradation value,deterioration of image quality is reduced.

An embodiment of the present disclosure is an image forming systemincluding a classifying section that classifies a block formed by Nbpixels included in image data into Nt (Nt is an integer which satisfies2≤Nt<Nb) groups including a group including a pixel of a highestgradation value and a group including a pixel of a lowest gradationvalue, a positional information generation section that generates, forat least one of the Nt groups, positional information that isinformation indicating positions of pixels that form the correspondinggroup, a block gradation value determination section that determines ablock gradation value in accordance with a data amount that is smallerthan a data amount that is used for expressing gradation values of theNb pixels for each of the Nt groups as a target, based on a gradationvalue for each of pixels that form each group, a tentative determinationsection that tentatively determines, for each of the Nt groups as atarget, a dot arrangement that is information indicating whether or nota dot is formed for each of pixels that form the block, based on theblock gradation value, a dot arrangement determination section thatdetermines, for the block as a target, the dot arrangement using thetentatively determined dot arrangement and the positional information,and an image forming section that forms an image using the determineddot arrangement. According to this embodiment, the dot arrangements,each of which has been tentatively determined for the corresponding oneof the groups, are combined using the positional information, andtherefore, deterioration of image quality is reduced.

In the above-described embodiment, the block gradation valuedetermination section may be configured to determine the block gradationvalue of each of the Nt groups, based on a representative value ofgradation values stored in pixels that form the corresponding group.According to this embodiment, it is not necessary to determine a blockgradation value for each of the pixels that form the correspondinggroup, and therefore, a processing load is reduced.

In the above-described embodiment, the tentative determination sectionmay be configured to tentatively determine the dot arrangement using anorder of priories for forming a dot for the pixels that form the block.According to this embodiment, speed of tentatively determining a dotarrangement is increased by determining the order of priorities inadvance.

In the above-described embodiment, a total of a data amount of the blockgradation value and a data amount of the positional information may besmaller than a data amount of the dot arrangement for the Nb pixels.According to this embodiment, if the block gradation value and thepositional information are transferred, image forming speed is hardlyreduced due to transfer speed.

In the above-described embodiment, the classifying section may beconfigured so as not to execute, if an edge is not included in theblock, the classifying, and the dot arrangement determination sectionmay be configured so as to determine, if an edge is not included in theblock, the dot arrangement, based on the block gradation value.According to this embodiment, if an edge is not included in the block, aprocessing load is reduced and the data amount of data in accordancewith the block gradation value and the position information is reduced.

In the above-described embodiment, the image forming system may furtherincludes a determination section that determines whether or not an edgeis included in the block, based on gradation values stored in the pixelsthat form the block and comparison with a corresponding threshold thathas been determined in advance. According to this embodiment, thedetermining is performed based on comparison with the threshold that hasbeen determined in advance, and therefore, the determining may beexecuted in a simple manner.

The tentative determination section may be configured not to execute, ifan edge is not included in the block, the tentative determining, and theblock gradation value determination section may be configured to regard,if an edge is not included in the block, the Nt as 1. According to thisembodiment, the processing load is further reduced.

In the above-described embodiment, the positional information may beconfigured to indicate whether or not a pixel belongs to a group of theNt groups to which a representative pixel belongs, for other pixels thanthe representative pixel among the pixels that form the block. Accordingto this embodiment, a data amount of the positional information issmaller by a data amount of the representative pixel.

The present disclosure may be realized by various embodiments other thanthe above-described embodiment. For example, the present disclosure maybe realized by an embodiment of an image forming method, an imageforming device that realizes the method, a program that realizes themethod, a storage medium, not a temporary storage medium, which storesthe program, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a schematic diagram of an image forming system.

FIG. 2 is a flowchart illustrating compressed data transmissionprocessing.

FIG. 3 is a flowchart illustrating data compression processing.

FIG. 4 is a view illustrates blocks.

FIG. 5 is a flowchart illustrating image forming processing.

FIG. 6 is a flowchart illustrating halftone processing.

FIG. 7 is a graph illustrating a large, medium, and small dot densityconversion table.

FIG. 8 is a table illustrating a relationship between a representativevalue, a block gradation value, and the like.

FIG. 9 is a view illustrating a threshold applied to each pixel of ablock of interest.

FIG. 10 is a view illustrating dots of an H value group, which arearranged.

FIG. 11 is a view illustrating masking of the H value group.

FIG. 12 is a view illustrating dots of an L value group, which arearranged.

FIG. 13 is a view illustrating masking of the L value group.

FIG. 14 is a view illustrating how dot arrangements are combined.

FIG. 15 is a view illustrating gradation values of an ink color in imagedata (a second embodiment).

FIG. 16 is a view illustrating a relationship between gradation valuesand a density pattern (the second embodiment).

FIG. 17 is a view illustrating dots that occur when the gradation valueis 1.

FIG. 18 is a view illustrating dots that occur when the gradation valueis 2.

FIG. 19 is a view illustrating dots that occur when the gradation valueis 3.

FIG. 20 is a view illustrating dots that occur when the gradation valueis 14.

FIG. 21 is a view illustrating dots that occur when the gradation valueis 15.

FIG. 22 is a view illustrating dots that occur when the gradation valueis 16.

FIG. 23 is a view illustrating how dot arrangements are combined (thesecond embodiment).

FIG. 24 is a view illustrating a dot arrangement (a comparison example).

FIG. 25 is a schematic diagram of a printer (a third embodiment).

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Embodiment

FIG. 1 is a schematic diagram illustrating an outline of an imageforming system 20. The image forming system 20 includes a datacompression device 100 and a printer 200. The data compression device100 coverts image data and thus generates compressed data. Thecompressed data is intermediate data that is obtained in the course ofhalftone processing of the image data. The data compression device 100transmits the compressed data to the printer 200. The image formingsystem 20 includes the data compression device 100 and thus functions asa data compression system.

The data compression device 100 in this embodiment is a personalcomputer. The data compression device 100 includes a CPU 110 and astorage medium 120. In the storage medium 120, a compression LUT and aprinter driver are stored. The storage medium 120 includes an RAM andexecutes temporary storing for execution of the printer driver.

The printer 200 is an image forming device. The printer 200 includes aCPU 210, a storage medium 220, and an image forming mechanism 230. Inthe storage medium 220, a decompression LUT, a dither mask, and an imageforming processing program are stored. The storage medium 220 includesan RAM and executes temporary storing for execution of the image formingprocessing program.

The image forming mechanism 230 is a generic name of hardware used forink jet image formation. The image forming mechanism 230 includes aprint head or the like. A resolution of image formation performed by theprinter 200 is 1200 dpi. Ink colors used by the printer 200 are cyan(C), magenta (M), yellow (Y), and black (K). The printer 200 is able toselect a size of each of dots formed on a print medium from large,medium, and small.

FIG. 2 is a flowchart illustrating compressed data transmissionprocessing. The CPU 110 executes a stored program, and thereby, the datacompression device 100 is realized. This program is a printer driver.

The compressed data is generated from image data represented by RGBvalues. The resolution of the image data in this embodiment is 1200 dpi.For the image data, data indicating a gradation value is stored in eachpixel. A gradation value in this embodiment is an integer value of 0 to255. Therefore, the image data has, for each pixel, a data amount of 8bits for each of RGB, that is, a data amount of 24 bits.

When compressed data transmission processing is started, colorconversion is executed (S300). That is, the RGB values are converted toCMYK values of ink color components. Each of the CMYK values takes aninteger value of 0 to 255. Thereafter, similar processing isindependently performed for each ink color component. Subsequently, datacompressed processing is executed for each ink color component (S400),compressed data is transmitted (S498), and compressed data transmissionprocessing is terminated.

FIG. 3 is a flowchart illustrating single ink color component datacompression processing. First, one of unprocessed blocks is focused on(S410).

FIG. 4 illustrates blocks. Each of the blocks is a group of Nb (8 inthis embodiment) squares surrounded by a rectangle indicated by a thickline. Each of the squares indicates a pixel. A number stored in a pixelindicates a gradation value of an ink color (for example, black).

The blocks are used as pseudo-pixels in processing described below. Datacompression is performed using each block of each ink color as a unit.

Subsequently, whether or not an edge is included in a block (which willbe hereinafter referred to as a block of interest) which is focused on(S420). S420 in this embodiment is executed depending on whether or notan expression below is satisfied.

(Maximum gradation value−Minimum gradation value)≥Threshold  Expression1

The threshold is 100 in this embodiment.

Looking at a block B1 indicated by FIG. 4, the maximum gradation valueis 243, and the minimum gradation value is 240. Therefore, Expression 1is not satisfied and it is determined that an edge is not included. Onthe other hand, looking at a block B2, the maximum gradation value is255 and the minimum gradation value is 3. Therefore, Expression 1 issatisfied and it is determined that an edge is included.

If an edge is not included (NO in S420), a flag is set off (S430). Withthis flag set off, it is indicated that an edge is not included. Thedata amount of the flag is 1 bit.

Subsequently, a representative value of a block is determined (S435).The representative value in this embodiment is calculated using a totalsum average of gradation values included in the block. When merely theterm “average” is used below, the term means the total sum average. Forthe representative value, if the average includes a figure of a decimalpoint or less, the average is rounded off by a decimal point or less toobtain an integral value. For example, looking at the block B1, theaverage is 240.625, and therefore, the representative value is 241. Asdescribed above, the gradation values of the Nb pixels are representedby one representative value, and thereby, a data amount that indicates agradation value per block is compressed from 8 Nb bits to 8 bits.

Subsequently, based on the representative value, a block gradation valueis determined (S440). Conversion from the representative value to ablock gradation value is realized by referring to the compression LUT.However, the compression LUT has been prepared for each block position,and therefore, in S440, the compression LUT that corresponds to aposition of a block that is focused on is used.

The position of a block is a relative positional relationship with adither mask. Assuming that a size of the dither mask is 64 pixels×64pixels, in a normal systematic dither method, a threshold stored in thedither mask is used for each group of 64 pixels×64 pixels, and thereby,an arrangement of dots is determined. As described above, when it isassumed that a dither mask is used, which position in the dither maskeach block is arranged is determined. This position is called theposition of a block.

When the position of a block is determined, the threshold of thecorresponding dither mask is determined, and therefore, it is possibleto examine, for all of cases in which the representative value is 0 to255, a halftone result when the gradation value of each of all of pixelsin the block is a representative value in advance. The compression LUTis generated based on the examination result.

Steps of generating the compression LUT will be described. By referringto a large, medium, and small dot density conversion table, an ink colorcomponent: DATA of an integer value in a range of 0 to 255 is convertedto density data: L_DATA, M_DATA, and S_DATA in accordance with a dotsize, that is, large, medium, or small. FIG. 7 illustrates theabove-described large, medium, and small dot density conversion table bya graph. This conversion is represented by expressions as follows.

L_DATA=L_tbl(DATA)

M_DATA=M_tbl(DATA)

S_DATA=S_tbl(DATA)

Similar to ink color data, each of large density data, medium densitydata, and small density data takes an integer value in a range of 0 to255. Therefore, the data amount is 8 bits. If the use rate of small dotsis close to 100%, resistance to displacement of a dot impact position orthe like is reduced, and therefore, as illustrated in FIG. 7, themaximum density of small dots is kept around 60%.

Assume that a dither mask threshold (an integer in a range of 0 to 254)of an examination target pixel is THi. In this case, assuming thatLM_DATA=L_DATA+M_DATA and T_DATA=LM_DATA+S_DATA, a halftone result wheninput data is DATA is 4-valued into large, medium, small, or off inaccordance with the following idea.

If L_DATA>THi, a large dot is on.

If L_DATA≤THi<LM_DATA, a medium dot is on.

If LM_DATA≤THi<T_DATA, a small dot is on.

IF T_DATA≤THi, a dot is OFF.

When, for the dither mask threshold: TH0 to TH7 of 8 pixels in positionsin an examination target block, the halftone result of each of all ofcases in which the DATA value is 0 to 255 is examined in advance, thedot arrangement in the block when all of the pixels in the block haveall the same DATA value is clearly known.

FIG. 9 illustrates a threshold used for each pixel position of a blockof interest. The printer 200 stores a general dither mask used for thesystematic dither method. The thresholds illustrated in FIG. 9 indicatethresholds in part of the dither mask, which are used for the block ofinterest. Note that the contents of FIG. 9 are also used for describingdetermination of dot arrangement using the decompression LUT (which willbe described later).

In the example of FIG. 9, the dither mask thresholds in the positions inthe block of an examination target are TH0=2, TH1=166, TH2=241, TH3=19,TH4=101, TH5=114, TH6=8, and TH7=59.

Based on the examination result for this case, when input DATA increasesone by one from 0 to 255, how the dot arrangement in the block and acombination of the respective numbers of generated large, medium, andsmall dots in the corresponding block accordingly change is tabulated.The dot arrangement in the block is an on or off result of each oflarge, medium, and small dots for respective pixel positions of TH0 toTH7. Furthermore, a table illustrated in FIG. 8 is generated for theblock positions by counting, as a block gradation value, in which stepof change the dot arrangement is.

A part of the table illustrated in FIG. 8 which corresponds toREPRESENTATIVE VALUE to BLOCK GRADATION VALUE is a compression LUT. Byreferring to the compression LUT, a representative value may beconverted to a block gradation value.

FIG. 8 illustrates that, as the representative value increases one byone from 0 to 255, the block gradation value changes in 22 steps from 0to 21. In this case, if the representative value is a (one of values of0 to 255), it is indicated that a is input in all of the pixels in theblock. Also, if the block gradation value changes in 22 steps, it isindicated that, in the target block, there are 22 patterns ofcombinations of large, medium, and small dots. Furthermore, inaccordance with a method of this embodiment, the dot arrangement in theblock, which corresponds to the block gradation value, is determined tobe only one dot arrangement, and therefore, the dot arrangement itselfchanges in 22 steps.

The table illustrated in FIG. 8 may be used also for the purpose ofobtaining a final halftone result from the block gradation value. Thefinal halftone result is a dot arrangement in the block. In that case, acorrespondence relationship of the block gradation value with a dot onor off result for pixel positions of 4 pixels in each of upper and lowerarrays, that is, 4×2=8 pixels in total, may be referred to.

The compression LUT of this embodiment depends on the dither maskthresholds in the block, and therefore, is prepared for each position inthe block. In this case, the maximum value of the block gradation valuealso depends on the dither mask thresholds in the block, and therefore,might differ in every block.

For example, if an examination is similarly performed on a case in whichTH0=240, TH1=65, TH2=120, TH3=90, TH4=17, TH5=168, TH6=212, and TH7=30,the block gradation value changes in 20 steps from 0 to 19 and, when theblock gradation value is 19, the dot on or off result is large dot onfor all of 8 pixels in the block.

On the other hand, it is one combination of six large dots and onemedium dot that corresponds to the block gradation value 19 in theexemplary table of FIG. 8. As described above, a dot combination whichcorresponds to the block gradation value depends on the ditherthresholds in the block, and therefore, even when the block gradationvalue is the same, the halftone result might differ depending on aposition of the block.

The maximum value of the block gradation value depends on the large,medium, and small dot density conversion table illustrated in FIG. 7and, when a large, medium, and small dot density conversion table whichhas a complex characteristic is used, the maximum value of the blockgradation value might be large. However, if the number of pixels thatform one block is 8, when a large, medium, and small dot densityconversion table which has a practical characteristic is used, there isnot a case in which the maximum value of the block gradation valueexceeds 31. Therefore, it is sufficient to assign 5 bits as the blockgradation value.

If a case in which the maximum value of the block gradation valueexceeds 31 occurs, the dither mask or the large, medium, and smalldensity conversion table may be corrected, such that, as a result, themaximum value of the block gradation value is 31 or less to cope withthe case.

As has been described above, the block gradation value of thisembodiment may be expressed by a data amount of 5 bits. Because the dataamount of the representative value, which has been determined above, is8 bits, the data amount is further compressed. Therefore, the dataamount of the block gradation value is smaller than 8 Nb bits, which isused for expressing the gradation values of the Nb pixels. Furthermore,the data amount of the block gradation value is smaller than the dataamount (8 bits) which is used for expressing the gradation value of onepixel. As described above, if there is not an edge in the block, thedata amount of a flag is added to the compressed data, resulting in adata amount of 6 bits.

Even when a data amount is compressed in the above-described manner,large deterioration of quality of a printed material may be reduced.This will be described later in description of image forming processing.

On the other hand, if an edge is included (YES in S420), a flag is seton (S442). Next, pixels included in a block of interest are classifiedinto an H value group and an L value group (S445).

The H value group is a group that is formed by pixels (which will behereinafter referred to as H value pixels) each of which has a gradationvalue that is classified, when gradation values of a pixel group thatforms a block are divided into two groups of a high value and a lowvalue, into the high value. On the other hand, the L group is a groupthat is formed by pixels (which will be hereinafter referred to as Lvalue pixels) each of which has a gradation value that is classifiedinto the low value.

In this embodiment, as criteria for classification into the H valuegroup and the L value group, an average between the maximum gradationvalue and the minimum gradation value is used. For example, in the blockB2 illustrated in FIG. 4, an average between the maximum gradation value255 and the minimum gradation value 3 is 129. Therefore, 3 pixels of thegradation value 255 are classified into the H value group. The H valuegroup, as a matter of course, includes a pixel of the maximum gradationvalue. On the other hand, pixels of the gradation values 3, 5, 12, and15 are classified into the L value group. The L value group, as a matterof course, includes a pixel of the minimum gradation value. If thegradation value matches the average, the gradation value is classifiedinto the H value group in this embodiment. In another embodiment, thegradation value that matches the average may be classified into the Lvalue group.

Next, the positions of the H value pixels are stored (S450). Informationthat is stored in S450 will be hereinafter referred to as positionalinformation. As the positional information, 1, which indicates the Hvalue pixel, or 0, which indicates the L value pixel, is stored for eachpixel. Therefore, the data amount of the positional information is Nbbits.

Next, a representative value of the H value group is determined (S460).The representative value of the H group in this embodiment is calculatedas an average of the gradation values of the H value pixels.

Subsequently, a block gradation value of the H value group is determined(S470). A determination method in S470 is the same as that in S440.

Next, a representative value of the L value group is determined (S480).The representative value of the L value group in this embodiment iscalculated as an average of the gradation values of the L value pixels.

Subsequently, a block gradation value of the L value group is determined(S470). A determination method in S470 is the same as that in S440.

As describe above, if there is an edge, information of a flag,positional information, and two block gradation values is generated.Therefore, a total 1+2Db+Nb bits is achieved. Db indicates the dataamount of a block gradation value. In this embodiment, Db is 5 bits, andtherefore, the data amount of the block which includes an edge is 19bits.

After S440 or S490, whether or not processing has been performed on allof blocks is determined (S495). If there is an unprocessed block (NO inS495), the process returns to S410. If all of blocks have been processed(YES in S495), data compression processing is terminated. Data that hasbeen processed in the above-described manner is compressed data.

In compressed data, an average Dc2 of the data amount for every Nbpixels is represented by an expression below.

Dc2=Ink{(Db+1)+E(2Db+Nb+1)(1−E)}  Expression 2

Ink indicates the number of ink colors (4 in this embodiment) and Eindicates a percentage of blocks which do not include an edge. To give aspecific number as an example, assuming that E is 90%, Dc2 in Expression2 is 29.2 bits.

The data amount of image data is 24×8=192 bits for every 8 pixels.Therefore, a compression rate achieved by data compression processing isabout 6.6. The compression rate depends on each of values of Db, E, Nb,and Ink in accordance with Expression 2. Db, Nb, and Ink are designvalues. E is a value which depends on image data, a value of Nb, a shapeof a block, and a method for determining whether or not there is anedge.

As has been described with reference to FIG. 3, compressed data isgenerated by two-step compression including a step of determining arepresentative value and a step of converting the representative valueto a block gradation value. Therefore, it is understood that therepresentative value is a first compressed gradation value and the blockgradation value is a second compressed gradation value. It is alsounderstood that data including the representative value and thepositional information is first compressed data and data including theblock gradation value and the positional information is secondcompressed data. However, in this embodiment, when merely the term“compressed data” is used, the term means the second compressed data.

The compression rate of the first compressed data will be examinedbelow. In the first compressed data, an average value Dc1 of a dataamount per pixel group that forms one block is represented by anexpression below.

Dc1=Ink{(8+1)E+(2×8+Nb+1)(1−E)}  Expression 3

Assuming that E is 90%, Dc1 in Expression 3 is 42.4 bits. Therefore, thecompression rate of the first compressed data is about 4.5. Thecompression rate achieved by a first step compression depends on each ofvalues of E, Nb, and Ink in accordance with Expression 3.

When E=0 is substituted in Expression 3, Dc1=100 bits is achieved.Therefore, even when E=0, the above-described average Dc1 is smallerthan 196 bits. That is, even when E=0, the above-described average Dc1is compressed to be smaller than the data amount per pixel group thatforms one block in the image data.

Next, with reference to FIG. 5, image forming processing will bedescribed. With reception of compressed data as a trigger, image formingprocessing is executed by the CPU 210.

First, halftone processing (which will be described later) is executed(S500). Subsequently, with dot data obtained by halftone processing as atarget, interlace processing is executed (S800) and image formation isexecuted (S900).

FIG. 6 is a flowchart illustrating halftone processing. First, one ofunprocessed blocks is focused on (S510). Processing of S510 is similarprocessing to that of S410.

Subsequently, whether or not a flag is on is determined (S515). If aflag is off (NO in S515), a dot size combination is achieved from theblock gradation values of the block of interest (S520).

The printer 200 stores, among the contents of the table illustrated inFIG. 8, a relationship between the block gradation value and the dotsize combination for each of positions in the block. These are as awhole called decompression LUT.

When block gradation values and positions of a block are given, usingthe compression LUT, a combination of the respective numbers of dots ofdifferent sizes, that is, dot sizes, may be achieved.

Next, a dot arrangement in the block of interest is determined (S530)and the process proceeds to S700. In S530, the dot size combinationwhich has been achieved in S520 and a dither mask are used.

For example, when the dot size combination is three large dots, onemedium dot, and one small dot, large dots are arranged in pixels whichhave the smallest value, the second smallest value, and the thirdsmallest value of the values of the thresholds, a medium dot is arrangedin a pixel which has the next smallest value, and a small dot isarranged in a pixel which has the further next smallest value. In theexample of FIG. 9, large dots are arranged in pixels of thresholds 2, 8,and 19, a medium dot is arranged in a pixel of a threshold 59, and asmall dot is arranged in a pixel of a threshold 101.

Originally, the dither mask is used for determining whether or not a dotis formed by comparing a stored threshold and the gradation value ofeach pixel in image data to one another. In one view, it may beunderstood that the thresholds stored in the dither mask indicate theorder of priorities for dot formation. That is, it may be understoodthat the systematic dither method is a method in which a dot is formedpreferentially in a pixel having a smaller numerical value as thethreshold of the dither mask. In S530, this characteristic is used.

FIG. 10 illustrates dots arranged in a manner described above. Lindicates a large dot, M indicates a medium dot, and S indicates a smalldot. Note that the combination of three large dots, one medium dot, andone small dot does not exist in the table of FIG. 8. However, todescribe a most complex case in which large, medium, and small are on atthe same time, this combination has been given as an example.

One the other hand, when a flag of the block of interest is on (YES inS515), a dot size combination is acquired for each of the blockgradation value of the H value group and the block gradation value ofthe L value group (S540). An acquisition method is the same as that inS520.

Subsequently, for each of the H value group and the L value group, a dotarrangement is determined (S545). A determination method is the same asthat in S530.

Next, the dot arrangement of the H value group is masked usingpositional information (S550). To describe masking, it is assumed thatthe dot arrangement illustrated in FIG. 10 is the dot arrangement of theH value group. It is also assumed that the positions of the H valuegroup are three positions of the gradation value 255, which are includedin the block B2 illustrated in FIG. 4. The dot arrangement of the Hvalue group illustrated in FIG. 10 is a result that has been achieved byarranging dots using the thresholds illustrated in FIG. 9 when the dotsize combination is two large dots, one medium dot, and one small dot.

FIG. 11 is a view illustrating masking of the H value group. Asillustrated in FIG. 11, in the H value group, dots in pixels in which 1is stored in the positional information are left and dots in pixels inwhich 0 is stored in the positional information are deleted. As aresult, one large dot and one small dot remain. This operation is calledmasking in this embodiment.

Next, the dot arrangement of the L value group is masked using thepositional information (S555). To describe masking, it is assumed thatthe dot arrangement illustrated in FIG. 12 is a dot arrangement of the Lvalue group. It is also assumed that the positions of the L value groupare positions of other gradation values than the gradation value 255,which are included in the bock B2 illustrated in FIG. 4. The dotarrangement of the L value group illustrated in FIG. 12 is a result thathas been achieved by arranging dots using the thresholds illustrated inFIG. 9 when the dot size combination is no large dot, no medium dot, andthree small dots.

FIG. 13 is a view illustrating masking of the L value group. Asillustrated in FIG. 13, in the L value group, dots in pixels in which 0is stored in the positional information are left and dots in pixels inwhich 1 is stored in the positional information are deleted. As aresult, two small dots remain.

Subsequently, the dot arrangements of the H value group and the L valuegroup are combined (S560) and the process proceeds to S700. FIG. 14illustrates how the dot arrangements are combined.

As illustrated in FIG. 14, in a pixel in which a dot is arranged in oneof the dot arrangement of the H value group after masking and the dotarrangement of the L value group after masking, the dot is arranged asit is also after the dot arrangements have been combined. On the otherhand, in a pixel in which a dot is not arranged in either one of the dotarrangement of the H value group after masking and the dot arrangementof the L value group after masking, a dot is not arranged. Note that apixel in which a dot is arranged in both of the dot arrangement of the Hvalue group after masking and the dot arrangement of the L value groupafter masking does not exist because of the characteristic of mask.

In S700, whether or not processing for all of blocks is completed isdetermined. If there is an unprocessed block (NO in S700), the processreturns to S510. If processing for all of blocks is completed (YES inS700), halftone processing is terminated. In the above-described manner,dot data that indicates a dot arrangement of each ink color is obtainedfor all of pixels by halftone processing. The dot arrangement isinformation that indicates whether or not a dot is formed for eachpixel. The dot arrangement in this embodiment is information thatindicates that a dot size if it is determined that a dot is formed.

In the dot data, an average Dh of the data amount per pixel group thatforms one block is represented by an expression below. Ls in theexpression blow indicates an operation result obtained by forming aninteger by rounding up digits after the decimal point of log 2S.

Dh=Ink×Nb×Ls  Expression 4

In this embodiment, Dh=4×8×2=64 bits. Therefore, the data amount ofcompressed data is smaller than the data amount of the dot data,depending on an E value. Note that, normally, the E value in image datais 90% or more, and therefore, the data amount of compressed data issmaller than the data amount of the dot data.

According to this embodiment, at least advantages below may be achieved.

(a) A processing load for obtaining dot data from image data is small.This is because, after a representative value is determined, a dotarrangement may be determined or tentatively determined only byreferring to a compression LUT, a decompression LUT, and a dither mask.

(b) If there is an edge in a block, two dot arrangements which have beententatively determined are combined to determine a dot arrangement, andtherefore, deterioration of image quality is reduced.

(c) Determination of a block gradation value and acquisition of a dotarrangement using the block gradation value may be executed by the datacompression device 100 and the printer 200, respectively. Therefore, aprocessing load of the data compression device 100 may be reduced.Furthermore, an image forming speed is hardly reduced.

(d) Compressed data having a smaller data amount than that of image datais transferred, and therefore, a probability that a transfer speed is abottleneck of an image forming speed is reduced. If a transfer speed isa bottleneck of an image forming speed, the image forming speed isincreased.

(e) Compressed data is transferred, and therefore, only a small storagecapacity is used for spooling.

Second Embodiment

A second embodiment will be described with a focus on a different pointfrom the first embodiment described above. Contents that will be notparticularly described are the same as the corresponding contents in thefirst embodiment. Note that each of third and subsequent embodimentswill be also described with a focus on a different point from the firstembodiment described above and contents that will be not particularlydescribed are the same as the corresponding contents in the firstembodiment.

In the second embodiment, only one dot size is used. That is, S=2. Inthe second embodiment, the size of a block is 4 pixels×4 pixels=16pixels.

Data compression processing in the second embodiment will be described.In the second embodiment, a block gradation value is represented in 17gradations from 0 to 16. Therefore, the data amount of the blockgradation value is 5 bits.

FIG. 15 illustrates gradation values of an ink color in image data. FIG.15 illustrates one block extracted from the entire image data.

In the example of FIG. 15, the maximum gradation value is 210 and theminimum gradation value is 10, and therefore, it is determined thatthere is an edge (YES in S420). An average between the maximum gradationvalue and the minimum gradation value is 110. L value pixels arehatched. An average of gradation values of the H value group is 206.25.An average of gradation values of the L value group is 11.75.

Note that an average of gradation values of all of pixels is 109. Theaverage for all of the pixels will be used in description below forcomparison with a known method.

In S470 and S490, to convert each of the above-described averages to ablock gradation value represented in 17 gradations from 0 to 16, each ofthe values of the averages, which have been described above, ismultiplied by 16/255 to form an integer. However, when an integer isformed by simply rounding off or the like, a pseudo outline resultingfrom reproduction in only 17 gradations occurs.

In this embodiment, to reduce the pseudo outline as much as possible, arandom number: rand ( ) of 0 or more and less than 1 is generated foreach block and, after adding the random number to the average, aninteger is formed by rounding off by a decimal point or less. Forexample, assume that a rand ( ) value of this block is 0.324. Assumingthat an operator that is used for forming an integer by rounding off bya decimal point or less is written as INT ( ), the H value group and theL value group are calculated as follows.

H value group=INT(206.25× 16/255+0.324)=13

L value group=INT(11.75× 16/255+0.324)=1

Note that, when a similar calculation is performed using the average 109for all of the pixels, the following result is obtained.

The gradation value of the entire block INT(109× 16/255+0.324)=7

Next, halftone processing in this embodiment will be described. In thisembodiment, instead of S520 and S540, a dot arrangement is achievedusing a density pattern method.

The resolution of image formation performed by the printer 200 is 1200dpi, and therefore, instead of enabling reproduction in 17 gradationsusing the block gradation value, the resolution is reduced to ¼ both inrow and column directions. Therefore, image formation in whichreproduction in 17 gradations at 300 dpi is possible may be enabled.

FIG. 16 illustrates a relationship between gradation values and adensity pattern. A pixel in which a dot is formed is a pixel in which anumber of the block gradation value or less is stored. That is, in apixel in which a smaller number is stored, a dot is preferentiallygenerated. In this embodiment, a dot concentration type is employed.

FIG. 17 illustrates that, when the block gradation value is 1, a singledot is generated. A circle is marked in a pixel in which a dot isgenerated. The pixel in which a dot is generated is a pixel in which 1is stored as a threshold. Because a dot concentration type is employed,pixels in which 1 to 4 are stored as thresholds are pixels located in aninner side. The pixels located in an inner side are 4 pixels that arenot adjacent to another block.

FIG. 18 illustrates that, when the block gradation value is 2, two dotsare generated. A pixel in which a dot is generated is a pixel in which anumber 2 or less is stored as a threshold.

FIG. 19 illustrates that, when the block gradation value is 3, threedots are generated. A pixel in which a dot is generated is a pixel inwhich a number 3 or less is stored as a threshold.

FIG. 20 illustrates that, when the block gradation value is 14, 14 dotsare generated. A pixel in which a dot is generated is a pixel in which anumber 14 or less is stored as a threshold.

FIG. 21 illustrates that, when the block gradation value is 15, 15 dotsare generated. A pixel in which a dot is generated is a pixel in which anumber 15 or less is stored as a threshold.

FIG. 22 illustrates that, when the block gradation value is 16, 16 dotsare generated. A pixel in which a dot is generated is a pixel in whichthe number 16 or less is stored as a threshold. That is, a dot isgenerated in each of all of the pixels.

Note that it is understood that FIG. 16 is a view illustrating that,when the block gradation value is 0, a dot is not generated at all.

FIG. 23 illustrates how the dot arrangements of the H value group andthe L value group are combined (S560). A circle is marked in a pixel inwhich a dot is formed. A pixel which is masked is hatched.

FIG. 24 illustrates, as a comparison example, a dot arrangement when thegradation values of the entire block are used. When FIG. 15, FIG. 23,and FIG. 24 are compared to one another, in a dot arrangement accordingto this embodiment, image reproducibility is higher than that in the dotarrangement of the comparative example.

Third Embodiment

FIG. 25 illustrates an outline of a printer 20 a. The printer 20 aexecutes color conversion (S300) and a data compression program (S400),and thereby, functions in stand-alone, similarly to the image formingsystem 20. Thus, the printer 20 a forms an image forming system in abroad sense. Furthermore, the printer 20 a functions as a datacompression device or a data compression system. Note that a datacompression processing program and an image forming processing programare collectively called image forming program.

Fourth Embodiment

In this embodiment, the data amount of positional information is Nb−1bits. Even when the positional information of this embodiment is used,image formation may be executed in a similar manner to that described inthe first embodiment.

An outline of the positional information of this embodiment is asfollows. After one block is divided into the H value group and the Lvalue group, the same group as that of a pixel position 0 and adifferent group from that of the pixel position 0 are handled as a G0group and a G1 group, respectively. As a representative pixel, the pixelposition 0 is determined to be a proper pixel position, such as a pixellocated in an upper left part in a block or the like, in advance. If itis stored in the printer 200 in advance that the pixel position 0belongs to the G0 group, it is not necessary to include informationabout the pixel position 0 in the positional information.

Note that the positional information in this embodiment does notdirectly indicate information (which will be hereinafter referred to asidentification information) which indicates which one of the G0 groupand the G1 group is the H value group and which one of the G0 group andthe G1 group is the L value group. However, even when the identificationinformation is not indicated, combining (S560) of dot arrangements inhalftone processing may be executed. Specifically, for one of the G0group and the G1 group, as described as S550 of the first embodiment,when a pixel which belongs to the other group is masked, the same resultas that in the first embodiment may be achieved.

In the first embodiment, the identification information is included inthe positional information and also it is not necessary to mask the Lvalue group, and therefore, a step of masking a dot arrangement usingthe L value group is not provided. As a matter of course, in the firstembodiment, masking for the L value group may be executed, too.Although, in the description of the first embodiment, the positionalinformation has been described as information that indicates theposition of an H value pixel, as a matter of course, the positionalinformation is also information that indicates the position of an Lvalue pixel.

Note that, also in this embodiment, the H value group and the L valuegroup may be distinguished from one another and different processing maybe executed on each of the H value group and the L value group. Evenwhen the identification information is not included in the positionalinformation, the identification information may be obtained from theblock gradation value. That is, as illustrated in FIG. 8, a relationshipbetween the block gradation value and the representative value in thisembodiment is a relationship of monotonous increase in a broad sense,and therefore, it is understood that, when the block gradation value ofG0 and the block gradation value of G1 are compared to one another, ifthe former is larger, the G0 group is the H value group, and if thelatter is larger, the G1 group is the H value group.

Fifth Embodiment

In this embodiment, it is not distinguished whether or not there is anedge in a block. That is, in data compression processing, determinationof S420 is eliminated and S445 to S490 are executed for all of blocks.Therefore, a flog is not needed. In halftone processing, determinationin S515 is eliminated and S540 to S560 are executed for all of blocks.

Note that, assuming that all of pixels in the block have the samegradation value, if a pixel in which the gradation value is an averagevalue or more is classified in the H value group in S445, all of pixelsare classified into the H value group, and therefore, subsequentprocessing for the L value group may be omitted. If not omitted, the dotarrangement of the L value group is not reflected in any pixel whencombining the dot arrangements of the H value group and the L valuegroup, and therefore, the representative value of the L value group maybe processed as a proper value, that is, for example, 0 or the like.

In the compressed data in this embodiment, the average Dc2 of the dataamount per image group that forms one block is represented by anexpression below.

Dc2=Ink(2Db+Nb)  Expression 5

When the same numerical value as that in the first embodiment issubstituted, 4×(2×5+8)=72 bits. As described above, the compressed datais compressed to be smaller than image data, but is larger than 64 bits,which is an average Dh of the data amount for dot data. Therefore, as amodified example of the fifth embodiment, processing up to generation ofdot data may be executed by the data compression device 100 andgenerated dot data may be transmitted to the printer 200.

However, when the data compression device 100 executes processing up togeneration of dot data, a processing load of the data compression device100 is excessively increased, and thereby, a time which it takes toobtain a printed material might be increased. In contrast, in thisembodiment, similar to the first embodiment, advantages that areachieved by transferring compressed data may be enjoyed. The advantagesthat are achieved by transferring compressed data are mainly (c), (d),and (e), which have been described in the first embodiment.

Note that, as is evident from Expressions 4 and 5, a size relationbetween the data amount Dc2 and the average Dh depends on values of Dband Nb. If an expression below is satisfied, similar to the firstembodiment, compressed data has a smaller data amount than that of dotdata.

NbLs>2Db+Nb ↔Ls−1>2Db/Nb  Expression 6

In accordance with Expression 6, when S=2, that is, when only one dotsize is used, the inequality does not hold. Assuming S>2, for example,when S=4 is substituted in Expression 6, Nb>2Db. Therefore, for example,when Nb is 12 pixels (for example, 3 pixels×4 pixels) and Db is 5 bits,the inequality holds.

On the other hand, in a relationship between the average Dc1 of the dataamount in the first compressed data and the data amount Dh in thisembodiment, a condition under which Dh>Dc1 holds is represented by anexpression below.

NbLs>2×8+Nb ↔Ls−1>16/Nb  Expression 7

For example, when S=4 is substituted in Expression 7, Nb>16. Therefore,for example, when Nb is 18 (for example, 3 pixels×6 pixels) pixels, theinequality holds.

In this embodiment, as compared to the first embodiment, there is anadvantage that conditional branches are reduced and processing issimplified by processing all of blocks in the same manner. Particularly,when processing is incorporated in hardware and thus processing of the Lvalue group and processing of the H value group are executed inparallel, or the like, speed reduction is not caused and this embodimentis effective.

Note that a combination of the method of the first embodiment which isillustrated in FIG. 3 and in which data compression processing isperformed by performing different processing between a block in whichthere is an edge and a block in which there is not an edge and themethod of the fifth embodiment in which halftone processing is performedassuming that all of blocks are blocks in which there is an edge may beemployed. In that case, for a block in which there is not an edge, inhalftone processing, all of pixels may be regarded as the H value groupand the representative value of the L value group may be caused to be aproper value, that is, 0 or the like. In accordance with this method,both of a high compression rate according to the first embodiment and asimple method that is suitable for incorporation in hardware accordingto the fifth embodiment may be achieved at the same time.

The present disclosure is not limited to the embodiments and themodified examples disclosed herein, but may be realized in various formswithout departing from the gist of the present disclosure. For example,some or all of the technical features described in the embodiments andthe modified examples, which correspond to technical features in aspectsdescribed in SUMMARY, may be replaced or combined with some or all ofthe technical features in order to solve some or all of the problemsdescribed above. Unless the technical features are described asessential technical features herein, the technical features may bedeleted as appropriate. For example, the following are examples of thetechnical features.

When a plurality of block gradation values is determined for the sameblock, the number of values that are to be determined may be larger thantwo, which corresponds to the H value group and the L value group.However, the number Nt of values that are to be determined is preferablysmaller than Nb (the number of pixels that form one block). That is,Nb>Nt is preferable. For example, when Nb=8, the number Nt of values maybe fixed to Nt=3, and also, the number Nt of values may be caused todiffer between when Nt=2 and when Nt=3 in accordance with the gradationvalue of image data in a block.

As described above, when Nt is changed to 3 or more, Expression 2 isgeneralized as follows. Ln in an expression below indicates an operationresult obtained by forming an integer by rounding off by a decimal pointor less of log₂ Nt.

Dc2=Ink{(Db+1)E+(NtDb+LnNb+1)(1−E)}  Expression 2A

Similarly, Expression 3 is generalized as follows. In an expressionbelow, the data amount (8 bits in the first embodiment) that expressesgradations of an ink color is written as Di.

Dc1=Ink{(Di+1)E+(NtDi+LnNb+1)(1−E)}  Expression 3A

Similarly, Expression 5 is generalized as follows.

Dc2=Ink(NtDb+LnNb)  Expression 5A

Similarly, Expression 6 is generalized as follows.

NbLs>NtDb+LnNb ↔Ls−Ln>NtDb/Nb  Expression 6A

Similarly, Expression 7 is generalized as follows.

NbLs>NtDi+LnNb ↔Ls−Ln>NtDi/Nb  Expression 7A

For comparison with image data, the following applies. Assuming that thedata amount per pixel in image data is Dp (24 in this embodiment) bits,if, when whether or not there is an edge in a block is notdistinguished, an expression below is satisfied, compressed data has asmaller data amount than that of the image data.

DpNb>Ink(NtDb+LnNb)  Expression 8

If, when whether or not there is an edge in a block is notdistinguished, an expression below is satisfied, data obtained bycompression in the first step has a smaller data amount than that of theimage data.

DpNb>Ink(NtDi+LnNb)  Expression 9

In addition to the above-described modified example, another modifiedexample will be described.

Data that is obtained by compression in the first step may be a transfertarget. That is, the representative values of the H value group and theL value group and the positional information may be transferred. Whenthis modified example is applied to the fourth embodiment, as a matterof course, the printer 200 is able to obtain identification informationalso by using representative values.

A method for determining whether or not there is an edge and a methodfor classifying a pixel into the H value group or the L value group maybe changed as appropriate. For example, as a method for classifying apixel into the H value group or the L value group, a fixed value (agradation value 128) may be used as a threshold.

A representative value may be determined by some other method than amethod using the total sum average. For example, a geometrical mean, amedian, or a most frequent value may be used. As another option, ahighest gradation value in the block may be used as the representativevalue of the H value group, and a lowest gradation value in the blockmay be used as the representative value of the L value group.

Instead of determining, after determining the representative value, theblock gradation value, the block gradation value may be obtained per aplurality of pixels, then, the representative value of a plurality ofblock gradation values may be determined, and thereby the respectiveblock gradation values of the H value group and the L value group may bedetermined.

A dot arrangement is tentatively determined from a combination of thenumbers of dots (S530, S540), and therefore, there may be a case inwhich a dither mask or the density pattern method is not used. Forexample, dots may be arranged at random.

Based on the block gradation value, a method for tentatively determininga dot arrangement (S540, S550 in the first embodiment) may be changed.For example, by referring to a LUT once, a dot arrangement may betentatively determined from the block gradation value. The LUT isprepared for each of positions in a block and corresponds to a part ofFIG. 8, which corresponds to the REPRESENTATIVE VALUE, UPPER 4 PIXEL DOTARRANGEMENT, and LOWER 4 PIXEL DOT ARRANGEMENT, and a block gradationvalue is associated with a dot arrangement. In this modified example,although the data amount of a decompression LUT is increased, the speedof processing performed for a dot arrangement is increased.

As another method for tentatively determining a dot arrangement, byreferring to a LUT once, the block gradation value may be converted toon dot number correspondence data which indicates a combination of ondot numbers of the large, medium, and small dot sizes, the respective ondot numbers of the large, medium, and small dot sizes may be obtainedfrom the on dot number correspondence data, and then, S550, which hasbeen described as an embodiment, may be executed. When Nb is 8, thereare 165 combinations of the respective on dot numbers of three types ofdots, that is, large, medium, and small dots. When an on dot numbercorrespondence LUT in which the on dot number correspondence data from 0to 164 and 165 different on dot number combinations are associated withone another in one-to-one correspondence is generated, the on dot numbercorrespondence data may be restored to the on dot number data byreferring to the on dot number correspondence LUT. The on dot numbercorrespondence data may be stored with 8 bits, and therefore, the datasize may be reduced as compared to a case in which the on dot numberdata of the three types of dots, that is, the large, medium, and smalldots, is directly described.

The data amount of the block gradation value may be changed, asappropriate. For example, the data amount of the block gradation valuemay be changed in accordance with the number of pixels which form ablock. When 16 pixels are included in one block, in almost all cases,the maximum value of the block gradation value is 63 or less. In thiscase, the data amount of the block gradation value is sufficient with 6bits. Also, when 4 pixels are included in one block, in almost allcases, the maximum value of the block gradation value is 15 or less. Inthis case, the data amount of the block gradation value is sufficientwith 4 bits.

Although, in the first embodiment, after converting all of blocks tosecond compressed data, halftone processing in which a final dotarrangement is obtained is executed, a processing unit for theseprocesses may be set to be a smaller block number. For example, afterperforming conversion to second compressed data on units of all ofblocks in a row direction and one block in a column direction, halftoneprocessing may be executed. Also, in this case, the advantages (a) to(e), which have been described in the first embodiment, are achieved.

Conversion to compressed data and halftone processing may besubsequently executed for every block as a processing unit. In thiscase, although the advantages (c) to (e) are lost, the advantages (a)and (b) are still achieved, and therefore, a sufficiently usefultechnology may be achieved.

Masking of the L value group may be omitted. In this case, the dotarrangement of the H value group after masking and the dot arrangementof the L value group which has not been masked are combined. Thus, aprocessing load is reduced. In a pixel in which a dot is arranged bothin the dot arrangement of the H value group after masking and the dotarrangement of the L value group, a dot of an H value after masking isarranged. However, conditions of this method are that a continuousdither method is used for halftone processing and that the total numberof the large, medium, and small dots monotonously increases relative tothe block gradation value.

In each of the above-described embodiments, some or all of functions anda part or the whole of processing that are realized by software may berealized by hardware. Also, some or all of functions and a part or thewhole of processing that are realized by hardware may be realized bysoftware. As hardware, for example, various types of circuits, such asan integral circuit, a discrete circuit, a circuit module in which thesecircuits are combined, or the like, may be used.

As described in the first embodiment, in an embodiment in which themethod for determining a dot arrangement is changed based on whether ornot there is an edge, it may be temporarily regarded that there is anedge for all of blocks, a dot arrangement may be tentatively determinedfor each of the H value group and the L value group, and thereafter, ifall of the pixels in the block belong to one of the H value group andthe L value group, a dot arrangement may be determined by ignoring atentative determination result for the other one of the H value groupand the L value group to which no pixel belongs at all. Such a method ispractical when the method is realized by hardware.

This application claims priority under 35 U.S.C. § 119 to JapanesePatent Application No. 2017-012912, filed Jan. 27 2017. The entiredisclosure of Japanese Patent Application No. 2017-012912 is herebyincorporated herein by reference.

What is claimed is:
 1. An image forming system comprising: a classifyingsection that classifies a block formed by Nb pixels included in imagedata into Nt (Nt is an integer that satisfies 2≤Nt<Nb) groups includinga group including a pixel of a highest gradation value and a groupincluding a pixel of a lowest gradation value; a positional informationgeneration section that generates, for at least one of the Nt groups,positional information that is information indicating positions ofpixels that form the corresponding group; a block gradation valuedetermination section that determines a block gradation value inaccordance with a data amount that is smaller than a data amount that isused for expressing gradation values of the Nb pixels for each of the Ntgroups as a target, based on a gradation value for each of pixels thatform each group; a tentative determination section that tentativelydetermines, for each of the Nt groups as a target, a dot arrangementthat is information indicating whether or not a dot is formed for eachof pixels that form the block, based on the block gradation value; a dotarrangement determination section that determines, for the block as atarget, the dot arrangement using the tentatively determined dotarrangement and the positional information; and an image forming sectionthat forms an image using the determined dot arrangement.
 2. The imageforming system according to claim 1, wherein the block gradation valuedetermination section determines the block gradation value of each ofthe Nt groups, based on a representative value of gradation valuesstored in pixels that form the corresponding group.
 3. The image formingsystem according to claim 1, wherein the tentative determination sectiontentatively determines the dot arrangement using an order of prioriesfor forming a dot for the pixels that form the block.
 4. The imageforming system according to claim 1, wherein a total of a data amount ofthe block gradation value and a data amount of the positionalinformation is smaller than a data amount of the dot arrangement for theNb pixels.
 5. The image forming system according to claim 1, wherein, ifan edge is not included in the block, the classifying section does notexecute the classifying, and if an edge is not included in the block,the dot arrangement determination section determines the dotarrangement, based on the block gradation value.
 6. The image formingsystem according to claim 5, further comprising: a determination sectionthat determines whether or not an edge is included in the block, basedon gradation values stored in the pixels that form the block andcomparison with a corresponding threshold that has been determined inadvance.
 7. The image forming system according to claim 5, wherein, ifan edge is not included in the block, the tentative determinationsection does not execute the tentative determining, and if an edge isnot included in the block, the block gradation value determinationsection regards the Nt as
 1. 8. The image forming system according toclaim 1, wherein the positional information indicates whether or not apixel belongs to a group of the Nt groups to which a representativepixel belongs, for other pixels than the representative pixel among thepixels that form the block.
 9. An image forming device comprising: aclassifying section that classifies a block formed by Nb pixels includedin image data into Nt (Nt is an integer that satisfies 2≤Nt<Nb) groupsincluding a group including a pixel of a highest gradation value and agroup including a pixel of a lowest gradation value; a positionalinformation generation section that generates, for at least one of theNt groups, positional information that is information indicatingpositions of pixels that form the corresponding group; a block gradationvalue determination section that determines a block gradation value inaccordance with a data amount that is smaller than a data amount that isused for expressing gradation values of the Nb pixels for each of the Ntgroups as a target, based on a gradation value for each of pixels thatform each group; a tentative determination section that tentativelydetermines, for each of the Nt groups as a target, a dot arrangementthat is information indicating whether or not a dot is formed for eachof pixels that form the block, based on the block gradation value; a dotarrangement determination section that determines, for the block as atarget, the dot arrangement using the tentatively determined dotarrangement and the positional information; and an image forming sectionthat forms an image using the determined dot arrangement.
 10. An imageforming method comprising: classifying a block formed by Nb pixelsincluded in image data into Nt (Nt is an integer that satisfies 2≤Nt<Nb)groups including a group including a pixel of a highest gradation valueand a group including a pixel of a lowest gradation value; generating,for at least one of the Nt groups, positional information that isinformation indicating positions of pixels that form the correspondinggroup; determining a block gradation value in accordance with a dataamount that is smaller than a data amount that is used for expressinggradation values of the Nb pixels for each of the Nt groups as a target,based on a gradation value for each of pixels that form each group;tentatively determining, for each of the Nt groups as a target, a dotarrangement that is information indicating whether or not a dot isformed for each of pixels that form the block, based on the blockgradation value; determining, for the block as a target, the dotarrangement using the tentatively determined dot arrangement and thepositional information; and forming an image using the determined dotarrangement.